diff --git a/src/constants.ts b/src/constants.ts
new file mode 100644
index 0000000..a9ddbd8
--- /dev/null
+++ b/src/constants.ts
@@ -0,0 +1,5 @@
+export const cssClassNames = {
+    loaded: 'ng-lazyloaded',
+    loading: 'ng-lazyloading',
+    failed: 'ng-failed-lazyloaded',
+}
diff --git a/src/lazyload-image.directive.ts b/src/lazyload-image.directive.ts
index 0da45be..3051844 100644
--- a/src/lazyload-image.directive.ts
+++ b/src/lazyload-image.directive.ts
@@ -17,8 +17,7 @@ import {
 } from '@angular/core';
 import { getScrollListener } from './scroll-listener';
 import { lazyLoadImage } from './lazyload-image';
-
-const windowTarget = typeof window !== 'undefined' ? window : undefined;
+import { isWindowDefined } from './utils';
 
 interface LazyLoadImageDirectiveProps {
     lazyImage: string;
@@ -45,7 +44,6 @@ export class LazyLoadImageDirective implements OnChanges, AfterContentInit, OnDe
     private propertyChanges$: ReplaySubject<LazyLoadImageDirectiveProps>;
     private elementRef: ElementRef;
     private ngZone: NgZone;
-    private platformId: string;
     private scrollSubscription;
 
     constructor(el: ElementRef, ngZone: NgZone) {
@@ -69,10 +67,8 @@ export class LazyLoadImageDirective implements OnChanges, AfterContentInit, OnDe
     ngAfterContentInit() {
         /**
          * Disable lazy load image in server side
-         * @see https://github.com/tjoskar/ng-lazyload-image/issues/178
-         * @see https://github.com/tjoskar/ng-lazyload-image/issues/183
          */
-        if (typeof window === 'undefined') {
+        if (!isWindowDefined()) {
             return null;
         }
 
@@ -81,6 +77,7 @@ export class LazyLoadImageDirective implements OnChanges, AfterContentInit, OnDe
             if (this.scrollObservable) {
                 scrollObservable = this.scrollObservable.startWith('');
             } else {
+                const windowTarget = isWindowDefined() ? window : undefined;
                 scrollObservable = getScrollListener(this.scrollTarget || windowTarget);
             }
             this.scrollSubscription = this.propertyChanges$
diff --git a/src/lazyload-image.ts b/src/lazyload-image.ts
index ad08180..957bd87 100644
--- a/src/lazyload-image.ts
+++ b/src/lazyload-image.ts
@@ -8,6 +8,7 @@ import 'rxjs/add/observable/of';
 import { Observable } from 'rxjs/Observable';
 import { getScrollListener } from './scroll-listener';
 import { Rect } from './rect';
+import { cssClassNames } from './constants';
 
 export function isVisible(element: HTMLElement, threshold = 0, _window: Window, scrollContainer?: HTMLElement) {
     const elementBounds = Rect.fromElement(element);
@@ -110,16 +111,16 @@ function setLoadedStyle(element: HTMLImageElement | HTMLDivElement) {
     const styles = element.className
         .split(' ')
         .filter(s => !!s)
-        .filter(s => s !== 'ng-lazyloading');
-    styles.push('ng-lazyloaded');
+        .filter(s => s !== cssClassNames.loading);
+    styles.push(cssClassNames.loaded);
     element.className = styles.join(' ');
     return element;
 }
 
 export function lazyLoadImage(element: HTMLImageElement | HTMLDivElement, imagePath: string, defaultImagePath: string, errorImgPath: string, offset: number, useSrcset: boolean = false, scrollContainer?: HTMLElement) {
     setImageAndSourcesToDefault(element, defaultImagePath, useSrcset);
-    if (element.className && element.className.includes('ng-lazyloaded')) {
-        element.className = element.className.replace('ng-lazyloaded', '');
+    if (element.className && element.className.includes(cssClassNames.loaded)) {
+        element.className = element.className.replace(cssClassNames.loaded, '');
     }
 
     return (scrollObservable: Observable<Event>) => {
@@ -131,7 +132,7 @@ export function lazyLoadImage(element: HTMLImageElement | HTMLDivElement, imageP
             .map(() => true)
             .catch(() => {
                 setImageAndSourcesToError(element, errorImgPath, useSrcset);
-                element.className += ' ng-failed-lazyloaded';
+                element.className += ' ' + cssClassNames.failed;
                 return Observable.of(false);
             })
             .do(() => setLoadedStyle(element));
diff --git a/src/scroll-listener.ts b/src/scroll-listener.ts
index a643ea1..b7a2c12 100644
--- a/src/scroll-listener.ts
+++ b/src/scroll-listener.ts
@@ -3,6 +3,7 @@ import 'rxjs/add/operator/sampleTime';
 import 'rxjs/add/operator/share';
 import 'rxjs/add/observable/empty';
 import { Observable } from 'rxjs/Observable';
+import { isWindowDefined } from './utils';
 
 const scrollListeners = new WeakMap<any, Observable<any>>();
 
@@ -17,7 +18,7 @@ export function sampleObservable(obs: Observable<any>, scheduler?: any) {
 // Typical, there will only be one observable per application
 export const getScrollListener = (scrollTarget): Observable<any> => {
     if (!scrollTarget || typeof scrollTarget.addEventListener !== 'function') {
-        if (typeof window !== 'undefined') {
+        if (isWindowDefined()) {
             console.warn('`addEventListener` on ' + scrollTarget + ' (scrollTarget) is not a function. Skipping this target');
         }
         return Observable.empty();
diff --git a/src/utils.ts b/src/utils.ts
new file mode 100644
index 0000000..4c51a90
--- /dev/null
+++ b/src/utils.ts
@@ -0,0 +1,3 @@
+export function isWindowDefined() {
+    return typeof window !== 'undefined';
+}